Printing from Firefox causes a "Print Preview Error" popup box with the message "an error occurred while printing...", and the resulting page is blank - whether printing to pdf or normal printer. NB I'm not trying to print preview but assume that is a background process.

UPDATE:
Open Firefox outside Sandboxie
type about:config and accept the risk
search sandbox
Switch security.sandbox.content.level to 2 (mine was 3, if needed switch it to 1 or 0 ).
Close Firefox
Relaunch it Sandboxed and re-try printing.

Once Firefox is released and we get the actual settings (betas tend to "play around with stuff sometimes"), we will be able to review this issue properly.
At this point, you are using sandbox content level 2, which is the same level used in the current Firefox release version.